You'll just need to check the automatic update box on the currency record. All that n/a means is that it doesn't have a downloaded rate for the date you're looking at.
If an exchange rate record has 1/1/1970 as the date, that just means that's the default rate for that currency and there haven't been any current rates downloaded yet.
Note: do NOT let the system use that default rate because most of them are wildly off.